Migrate babylon to @babel/parser

[danez]

Add option to switch back to legacy decorators parsing

This is a breaking change as it will by default only parse the latest decorators spec which changed the syntax.

[oktak]

Hello,
Will this migration would support dynamic import() ?

Issue:
I've got SyntaxError: 'import' and 'export' may only appear at the top level when I use import() within the React Component class definition.

I've seen that the error is raised from babylon\lib\parser\statement.js
And I see that there is similar error message in babel-parser too:
https://github.com/babel/babel/blob/master/packages/babel-parser/src/parser/statement.js#L151

I would like to ask if React-docgen could add the options for Parser that allowImportExportEverywhere could set to the value that would eliminate the SyntaxError mentioned above.

Thank you!

Version and Platform:

• react-docgen: 2.21.0 (npm view react-docgen version)
• node v8.9.4
• npm v5.6.0
• windows 7 64bit